The fabrication of complex semiconductor devices involves multiple processing steps which result in multiple patterned layers of different materials being applied to a substrate. The different layers overlie each other and must be accurately registered, or matched in position, to insure proper operation of the device. Displacement between corresponding features on different layers can degrade device performance or can cause the device to be totally inoperative. As used herein, "displacement" between layers of a semiconductor wafer refers to a displacement in the plane of the wafer. As semiconductor devices have become increasingly complex, the dimensions of the features have been correspondingly reduced. This reduction in feature dimensions has reduced acceptable tolerances on displacement between layers. When, for example, the minimum feature size is 2 micrometers, the registration error cannot exceed about 0.1 micrometer.
To assist in registration of overlying layers in semiconductor wafers, it has been common practice to include reqistration patterns or marks in each layer of the wafer. The patterns overlie each other and have a predetermined relationship when the layers are correctly registered. One commonly used registration pattern includes squares of different sizes on the layers to be registered. When the two layers are exactly registered, the squares are concentric. Any registration error produces a displacement of the squares relative to each other.
Since semiconductor wafers including multiple complex integrated circuits are expensive to fabricate, it is usually desirable to verify registration after the application of each layer to the wafer. If the displacement of the layers is outside tolerable limits, the defective layer can, in some cases, be removed and replaced with an accurately reqistered layer. In other cases, the wafer is scrapped, thereby saving the expense of further processing steps on defective wafers.
In the past, it has been common practice to verify registration manually. Experienced operators examine the regqistration of overlying patterns on each wafer. Such techniques are relatively slow and are subject to human error and contamination of the semiconductor wafers.
More recently, automated systems for measuring registration have been developed. In one highly successful registration measurement system, registration errors are measured optically. A video camera records an image of a set of registration patterns through a microscope. The image is processed to obtain a measurement of the registration error.
A measurement system unavoidably introduces certain errors into the measured values. The errors arise both in the optical and the electronic portions of the system and cannot be eliminated entirely. Typically, such errors are systematic, that is, the errors have the same magnitude and direction from measurement to measurement. In the past, it has been customary to calibrate such registration systems by comparing measurements with those obtained from another system, such as a scanning electron microscope, that is known to be accurate. Such calibration techniques are relatively complex and require additional expensive equipment.
It is a general object of the present invention to provide improved methods and apparatus for registration measurement.
It is another object of the present invention to provide methods and apparatus for measuring registration of patterns wherein the effect of systematic errors is eliminated.
It is a further object of the present invention to provide methods and apparatus for determining the systematic errors in a registration measurement system without requiring additional calibration equipment.
It is yet another object of the present invention to provide methods and apparatus for registration measurement which are easy to use.
It is still another object of the present invention to provide methods and apparatus for measuring registration between overlying layers of a semiconductor wafer with high accuracy.